The present invention generally relates to digital signal recording systems and reproducing apparatuses for reproducing signals recorded by such recording systems, and more particularly to a system for time-sequentially multiplexing a digital video signal of a component encoding system obtained by subjecting an analog video signal related to a color still picture information to digital pulse modulation, with a digital audio signal which is a main information and recording the time-sequentially multiplexed signal onto a rotary recording medium, and a reproducing apparatus for reproducing the signal recorded onto the rotary recording medium according to this recording system.
Recently, systems which record a digital video signal obtained by subjecting video and audio signals to digital pulse modulation such as pulse code modulation (PCM) and a digital audio signal onto a rotary recording medium (hereinafter simply referred to as a disc) as variations in geometrical configuration, and reproduce the recorded signal as variations in the intensity of light reflected from the disc or variations in electrostatic capacitance, have been developed and realized. Further, recording systems have been proposed for digital audio discs, according to which a digital video signal comprising color still picture information is added to a digital audio signal and recorded together on the same track on the disc. Generally, a plurality of music programs are recorded on the same side of such a digital audio disc, and the digital video signal comprising the color still picture information is recorded in correspondence with each of the recorded music programs. When reproducing such a digital audio disc, the music programs on the disc can be reproduced by a reproducing system which is common throughout the world. However, the television systems are not common throughout the world, and there roughly exist three kinds of television systems. Accordingly, in order to enable reproduction of the video signal recorded on the disc even if the television system employed in a region or country is different from the television system of the recorded video signal, it is first necessary to convert the recorded video signal into a signal format in accordance with the television system of the reproducing apparatus used in that region or country before obtaining a reproduced picture. The information content of the above digital video signal relates to a color still picture which helps the listener's imagination when he listens to the reproduced sounds of the digital audio signal. Hence, it is desirable to reproduce the digital video signal from the disc in the signal formats which are in accordance with each of the television systems, regardless of the differences in the television systems throughout the world.
The color television systems throughout the world can roughly be divided into three systems, that is, NTSC, PAL, and SECAM systems, according to the transmission formats of the chrominance signal. The horizontal scanning frequency of the NTSC system is 15.734 kHz while the horizontal scanning frequency of the PAL and SECAM systems are mainly 15.625 kHz, and the difference between the horizontal scanning frequencies does not become a big problem since the difference is only in the range of 0.7%. However, the number of scanning lines and the vertical scanning frequency in the NTSC system respectively are 525 lines and 59.94 Hz, while they are mainly 625 lines and 50 Hz in the PAL and SECAM systems. Accordingly, if the number of scanning lines of the recorded video signal is 525 lines, the number of scanning lines must be increased to 625 lines when reproducing the recorded video signal as a signal in accordance with the PAL system or the SECAM system, and results in the lack of information. Thus, in order to prevent this lack of information, the recorded video signal must have 625 scanning lines, and upon reproduction, the recorded video signal is reproduced as it is with the 625 scanning lines or the number of scanning lines is reduced to 525 lines.
In this case, the time required to transmit one frame of video signal having 625 scanning lines and the capacity of a frame memory circuit required for storing one frame of such a video signal, must respectively be larger than those required with respect to a video signal having 525 scanning lines. However, as described before, the video signal relates to still picture information and merely helps the listener with his imagination, and it is desirable for the reproduced picture to remain unchanged for a predetermined time rather than change instant by instant. Hence, with respect to the increase in the transmission time of the video signal, no inconveniences are introduced even when the transmission time increases by approximately 20%.
On the other hand, regarding the use of a frame memory circuit having a large capacity, a reproduced video signal of a higher quality can be obtained when obtaining a reproduced video signal in accordance with the
system or the SECAM system, compared to the case where the number of scanning lines of the video signal is increased and converted into 625 lines from 525 lines. However, such an effect cannot be obtained when obtaining a reproduced video signal in accordance with the NTSC system. On the other hand, because the video signal comprising the still picture information is transmitted at a low speed, a converting circuit for converting the number of scanning lines which is required to obtain the reproduced video signal in accordance with the NTSC system may be operated at a low speed, and there is no need to unnecessarily increase the capacity of the frame memory circuit.
In addition, when recording a digital video signal which is obtained by digitally modulating an analog video signal of a first television system onto the disc and reproducing the recorded digital video signal as an analog video signal of a second television system, the present inventors have previously proposed a system which obtains a digital video signal having a predetermined sampling frequency by setting a read-out speed from the memory circuit to a predetermined speed and then passing the digital video signal thus obtained through a digital-to-analog converter, in a Japanese Laid-Open Patent Application No. 57-160290 (Japanese Patent Application No. 56-46700). However, this previously proposed system was complex. Moreover, when the first television system was the NTSC system, there were problems in that the frequency band of the luminance signal became limited, the vertical resolution became low, and degradation was introduced in the reproduced picture.
In order to eliminate the above described problems of the previously proposed system, the present inventors have thus devised another digital signal recording system in a Japanese Patent Application No. 56-139567. According to this devised system, one frame (or one field) of the video signal related to the color still picture information comprises a luminance signal and two color difference signals (B-Y) and (R-Y), and three kinds of digital video signals are obtained by independently subjecting the luminance signal and the two color difference signals to digital pulse modulation. The three kinds of digital video signals are successively and time sequentially transmitted, and this devised system is in accordance with a component encoding system.
However, according to this devised digital signal recording system, one frame (or one field) of the digital luminance signal is recorded, one frame (or one field) of the first digital color difference signal is recorded thereafter, and one frame (or one field) of the second digital color difference signal is recorded after the first digital color difference signal. Hence, if the operator attempts to change the still picture being displayed when reproducing a disc recorded according to such a recording system, the two kinds of color information will successively change in terms of one picture after the luminance information of the still picture of one picture changes. As a result, there was a disadvantage in that the reproduced picture was irregular and unpleasant to watch.